This invention relates to a television signal transmission system which utilizes more than one transmission channel to send signals representing a high-definition widescreen image.
A conventional television receiver, such as a receiver in accordance with NTSC broadcast standards, has a 4:3 aspect ratio (the ratio of the width to the height of a displayed image). Recently, there has been interest in using higher aspect ratios for television receiver systems, such as 2:1, 16:9 or 5:3, since such higher aspect ratios more nearly approximate the aspect ratio of the human eye than does the 4:3 aspect ratio. Video images having aspect ratios of 5:3 and 16:9 have received particular attention since these ratios approximate the aspect ratio of many motion picture films. In addition, there has been interest in increasing the level of detail in the reproduced image above that of a conventional television image to more nearly approximate the level of detail found in motion picture film. However, television systems which transmit signals representing high-definition widescreen images should be designed to send a video signal which is compatible with existing television receivers since the widespread adoption of an incompatible system may be difficult.
A survey of several proposed high-definition widescreen television systems may be found in a paper by Robert Hopkins entitled "Advanced Television Systems", IEEE Transactions on Consumer Electronics, February 1988, pp. 1-15. Of the eight systems described in this article, five are compatible with existing NTSC television receivers to the extent that the transmitted signals may be received by a conventional NTSC receiver, without preprocessing by a converter, to produce a 4:3 image that is only slightly degraded relative to a standard NTSC image. These systems were developed by: AT&T Bell laboratories (the Bell system), Dr. William Glenn of the New York Institute of Technology (the Glenn system), the Del Ray Group (the Del Ray system), North American Philips (the NAP system) and a consortium of NBC and the David Sarnoff Research Center (the NBC system).
In the Bell system, two 6 MHz television channels are used to transmit signals representing a high-definition widescreen image. One channel carries a standard NTSC signal representing relatively low frequency luminance and chrominance signal components of the high-definition widescreen image. The second channel carries high frequency luminance and chrominance signals which may be used, by a special receiver, to augment the image represented by the NTSC signal.
The Glenn system also uses two channels, one of which carries an NTSC compatible signal. The second channel in the Glenn system uses only 3 MHz of the 6 MHz bandwidth and conveys image detail information which has been filtered to reduce its temporal resolution (i.e. to lower its frame rate). A frame store is used to combine the NTSC signal with the low-frame-rate high-detail signal. A widescreen effect is achieved in the Glenn system by increasing the vertical blanking interval while decreasing the horizontal blanking interval.
The Del Ray system uses only one 6 MHz channel which carries an NTSC compatible signal. This signal defines a high-definition image over six field periods. The high-definition image is reconstructed by a special receiver which combines the six fields in a frame store. The Del Ray system also increases the vertical blanking interval to achieve a widescreen effect.
The NAP system transmits a standard NTSC signal in one channel and an augmentation signal in a second channel. The augmentation signal is combined with the NTSC compatible signal by a special receiver to generate a widescreen, progressively scanned image. The horizontal detail of the image produced by the NAP system is substantially the same as that of an NTSC image.
The NBC system is the starting point for the invention set forth below. This system is described in greater detail in a paper by M. Isnardi et al. entitled "Decoding Issues in the ACTV System"; IEEE Transactions on Consumer Electronics, February, 1988, pp. 111-120. A portion of this system is also described in U.S. patent application Ser. No. 089,637 entitled "Apparatus for Processing High Frequency Information in a Widescreen Television System", which was allowed on Apr. 22, 1988. These references are hereby incorporated by reference. The system described in the referenced article and patent application uses a single 6 MHz channel to convey an augmented NTSC signal. When this signal is processed by a conventional NTSC receiver, a conventional television image is produced. However, when this signal is processed by a receiver which decodes and combines the various component signals, an extended definition, widescreen image is produced. The component signals which augment the basic NTSC signal are assigned to inefficiently used areas of the NTSC spectrum. Although this high-definition, widescreen image has more horizontal and vertical detail than a conventional television image and is overall very pleasing to the viewer, it may have less diagonal detail. Moreover, the overall level of detail is still less than that of motion picture film.